1. Field of the Invention
The present invention relates to a scanning optical system which is used in an information recording apparatus such as a laser beam printer.
2. Related Background Art
FIG. 1 shows a plan view of a scanning optical system of an information recording apparatus and FIG. 2 shows a sectional view thereof. A laser oscillator 2, a collimator lens 3 and a cylindrical lens 4 are secured to a side wall of an optical box 1 and a polygon mirror 5 is mounted therein. A spherical lens 6, a toric lens 7 and a reflection mirror 8 are sequentially arranged in a direction of outgoing light beam.
The laser beam emitted by the laser oscillator 2 passes through the collimator lens 3 and the cylindrical lens 4, is deflected by the polygon mirror 5, and it is f.theta.-corrected by the spherical lens 6 and the toric lens 7, and a resulting scanning light beam is reflected by the reflection mirror 8 and directed to a photoconductor drum which is an information recording medium.
In the prior art scanning optical system, the spherical lens 6 and the toric lens 7 are made of glass and the glass lenses are secured to the optical box 1 by ultraviolet ray cured adhesive so that the curing of the adhesive is promoted by irradiating an ultraviolet ray through the lens.
Recently, lens made of synthetic resin are frequently used in order to improve the productivity of the lenses, reduce the weight, reduce the cost and improve the performance by non-spherical planes. A preferable material of the synthetic resin is polycarbonate resin in view of good optical characteristic, small change of dimension by humidity and physical strength.
However, polycarbonate resin has a lower ultraviolet ray transmission than that of glass and it has only one fourth or fifth of the glass transmission. When it is to be secured by the ultraviolet ray cured adhesive as it is in the prior art glass lens, the following problem is encountered. The ultraviolet ray irradiation time is long and the workability is low. The adhesive is not uniformly cured due to ununiform irradiation. If the intensity of the ultraviolet ray is too high, the polycarbonate resin is deteriorated by a photo-oxidization reaction.